In Early Attempts to Identify and Isolate the Cause of Syphilis

In early attempts to identify and isolate the cause of syphilis, various bacteria in the discharge from chancres in experimental animals were isolated in pure culture. None of the bacteria, however, would cause the disease when used in attempts to infect healthy animals. What is a possible explanation for this observation?

A) The bacteria isolated from the animals were in fact the animal's normal microbiota and not pathogenic organisms; these bacteria would not cause disease in inoculated animals.
B) The bacteria thought to be isolated from the animals were actually skin bacteria from the people performing the experiments; these human bacteria would not cause disease in experimental animals.
C) The bacteria isolated from the animals were in fact the animal's normal microbiota; these bacteria would not cause disease in inoculated animals OR the bacteria thought to be isolated from the animals may have come from the people performing the experiments; these human bacteria would not cause disease in experimental animals.
D) The bacterial agent of syphilis causes disease in three stages; the organisms cultured from the first animals would be those that cause primary symptoms and would not be the same as those needed to cause disease in a second set of experimental animals.
E) When the causative organism of syphilis is removed from a host and cultured in the laboratory, it loses its capsule, without which it is unable to colonize a new host and will thus not cause disease OR the bacteria isolated from the animals were in fact the animal's normal microbiota; these bacteria would not cause disease in inoculated animals.